A monolithic convolver formed by providing a ZnO layer directly on a semiconductor Si surface is characterized by a relatively great in the TB product (T:signal processing time, B: bandwidth), has a high efficiency and is obtained at a low cost. These features lead to the possibility of using such convolver as a key device in a correlation signal processing apparatus of a spread-spectrum communication system.
However, such a monolithic convolver still involves the following drawbacks:
(i) A bias voltage must be applied to a metal gate on the ZnO layer in order to hold the Si surface in a depletion condition.
(ii) It takes a very long time constant to establish a constant condition of the Si surface after the bias voltage is applied, to the metal gate on the ZnO layer. A major reason for this is incompleteness of the ZnO layer.
(iii) The manufacturing yield is decreased by the process of providing the ZnO layer generally by sputtering, because the underlying Si substrate is known to be damaged by the plasma in the sputtering process.
(iv) The propagation loss of surface waves is large when they propagate along the ZnO layer surface on the Si substrate, and this invites a relatively large decrease in the efficiency.
(v) The Sezawa mode, although most often used because of its significantly large coupling coefficient, exhibits a relatively large speed dispersion of surface acoustic,, waves, and this nature necessarily limits the band width of the element.